1. Field of the Invention
The present invention generally relates to power generation equipment and systems. More particularly, this invention relates to a power generation plant that makes use of one or more induction generators in combination with one or more superconducting synchronous generators, the latter of which are operated to produce sufficient reactive power to meet the VAR requirements of the induction generators.
2. Description of the Related Art
Virtually all large turbine-driven generators used in the production of electrical power are synchronous generators. Synchronous generators generally comprise a rotor that serves as a source of magnetic lines of flux produced by a wound coil carried on the rotor, and a stator that comprises a number of conductors in which an alternating current is induced by the rotor as it rotates within the stator, generating a rotating magnetic field in the narrow airgap between the stator and rotor. Such generators are synchronous in that the rotor is rotated at a constant speed synchronous with the rotation of the magnetic field induced in the stator, thereby producing alternating current with a constant frequency (e.g., 60 Hz of the power grid). Synchronous generators are separately excited, and therefore do not require reactive power from the grid. However, because the rotor is driven by a prime mover (e.g., a turbine), controls are necessary to ensure synchronization of the rotor speed, voltage, phase shift and phase sequence with the magnetic field induced in the stator, and therefore the power grid.
Induction generators differ from synchronous generators in that the rotor comprises a number of conductors in which alternating current flow is induced as a result of the rotor being rotated at a speed higher than the rotating magnetic field of the stator. Induction generators are not self-exciting, in that the rotating magnetic field of the stator requires an external AC power source. While less efficient, induction generators have many advantages over synchronous generators, including simplicity, robustness, and cost. Induction generators are also less sensitive to speed variations and therefore can operate in a range of speeds. However, a major disadvantage of induction generators is that they must be supplied with reactive power, or VARs (volt-ampere reactive units). The VAR requirement can be reduced by using very small airgaps between the rotor and stator (e.g., a fraction of an inch), but such a constraint renders induction generators impractical for use in power generation of large power ratings and with large generator frame sizes (e.g., above 1 MVA).
To compensate for the VARs requirement, induction generators have been used in combination with capacitor banks or synchronous condensors, the latter of which are synchronous generators run unloaded to function solely for power factor correction of the induction generator. However, such approaches are cost-prohibitive for power generation plants. Induction generators have also been operated in combination with overexcited synchronous generators, wherein the excitation from the field winding is increased above what is needed to supply the required real power, thereby generating extra VARs. A drawback with this approach is that operating a synchronous generator in an overexcited mode produces large losses in the field winding, which increases the operating temperature of the field winding. As a result, the output of VARs during overexcited generator operation is limited by the field winding temperature rise. Alternative solutions have been proposed, such as U.S. Pat. No. 4,677,364 to Williams et al., U.S. Pat. No. 4,742,288 to Sugimoto et al., and U.S. Pat. No. 5,929,612 to Eisenhaure et al. Nonetheless, it is believed that the shortcomings and limitations of the above approaches are such that large induction generators are not currently used at power generation plants.